Medical injection needles are typically formed by continuously extruding a suitable metal, such as stainless steel, to form the shaft of the needle with a suitable outside diameter and an inner bore or lumen size also of prescribed diameter. The extruded material is cut to length and ground at one end to form a piercing point for insertion through the skin of a subject.
While such needles work fine for many applications where the requirements for injection flow rates and injection duration are not demanding, it is sometimes necessary, for a variety of reasons, to control the injection flow rate and/or injection duration or delivery time relatively precisely. For example, the injection duration can impact the pain associated with the injection, and slower flow rates or longer duration tend to be less painful. The flow rate and time of injection also may be significant to achieve better medical results of drug diffusion or absorption by the subject. There may also be practical factors such as patient mobility and age that impact the need for improved control of flow rate and/or injection time.
There are numerous factors or variables that can play a role in medical injection duration and flow rates. As examples only, drug delivery time and/or flow rates can vary with the size (diameter) of the needle shaft and the inside lumen, the viscosity of the drug, biologic or other injectable medicament, and the injection pressure. Even for a given medicament at a given temperature, the injection flow rate and duration can vary more than desired with standard injection needles.
One of the reasons for the variability is that the size of the lumen itself can vary significantly as between different batches of extruded needle shaft material. While the lumen is typically consistent within a given extrusion batch, it can vary significantly as between batches. Even the most carefully and precisely extruded needle material can vary significantly in lumen size—particularly as between different batches of needle shaft material. It is understood that due to the extrusion process itself, the outside diameter of the needle is much more accurately and predictably controlled than the inner diameter of the needle lumen. For example, the outer diameter may be controlled within about 0.001 inches.
It is the lumen size, however, that is particularly relevant to fluid flow, because even small variations can affect flow rates and injection duration. This is due to the physics of fluid flow through a lumen, as set forth, for example, in Poiseuille's Law. Poiseuille's law indicates that volume flow rate through a lumen is proportional to lumen radius taken to the fourth power (or R4), so small variations in the lumen size can make a large difference in flow rates at a given pressure and lumen length.
Accordingly, the one aspect of the present subject matter relates to injection needles, methods of making injection needles and injection devices employing such needles, which provide more accurate flow rate and injection duration, and reduce the flow variability as between needles fashioned from different extrusion batches, and potentially even as between individual needles of the same batch.